This invention relates to thin strip metal casting systems and more particularly to such systems which include an electromagnetic pump which subjects the liquid metal and an associated heat sink to a longitudinal electromagnetic field.
Over the past decade, a significant energy reduction in the steel making process has arisen from the use of continuous slab casting technology, where steel is cast directly from the melt. An improvement in rapid solidification has arisen for the production of thin strip known as melt spinning. Here, specimens are cast directly from the melt into strips having a thickness of about 0.254 to 1.27 mm (0.01 to 0.05 inches), using a conveyor or drum assembly chilled to below the solidification temperature, at belt or wheel peripheral speeds of about 23 meters/second. Rapid solidification, where heat is extracted from the strip by a cold, high conductivity wheel, is the preferred method of processing ferrous metals. The rate at which that strip is produced is determined by the rate of heat extraction. Even where the heat transfer is high, the liquid does not acquire the full conveyor velocity before it freezes, at which instance the specimen velocity is equal to that of the conveyor. The solidification region on the conveyor varies according to the conveyor linear speed for a given ribbon thickness. At the 23 meters/second speed, strip thicknesses of about 0.635 mm (25 mils) are practical at solidification lengths of 50 centimeters and wheel temperatures of 350.degree. K.
An electromagnetic pump of the polyphase, ac induction type, which may be used in a thin strip metal casting system, has, in a preferred arrangement, two primary members located above and below the main conveyor belt and metal ribbon specimen. Both the metal specimen, assumed to be non-ferromagnetic since the temperature is above the Curie temperature, and the metal chill block or belt form the secondary circuit for the induction of slip frequency currents. The synchronous field speed, v.sub.s, of the traveling wave set up by the two primary members is determined according to the relation: EQU V.sub.s =2.tau..sub.p f (1)
where .tau..sub.p is the pole pitch of the primary in meters and f is the excitation frequency in hertz. If the surface speed of the chill block, wheel or conveyor is V.sub.r, then the per unit slip is defined as: EQU S=(V.sub.s -V.sub.r)/V.sub.s ( 2)
for which it is understood that the frequency, f.sub.r, of the currents induced in the metal strip secondary and conveyor will always be less than or equal to the frequency of the excitation according to: EQU f.sub.r =sf (3)
In the case when the belt speed equals the primary field speed, slip equals zero and no currents are induced in the strip or belt transport. As the belt speed is reduced slightly from synchronous speed, current density builds up linearly with slip and power dissipation builds up as the square of the change in slip over the small slip range. Irrespective of the material resistivity, the basic efficiency, .eta., of the system is then equal to: EQU .eta.=1-s (4)
where if the total power, p.sub.t, is transmitted across the two air gaps into the secondary, then the quantity .eta.p.sub.t, is transformed into mechanical power and the quantity sp.sub.t is converted into a Joule loss for supplying the combined resistive loss of the chill block, p.sub.b, and strip specimen, p.sub.fe, as follows: EQU sp.sub.t =p.sub.b +p.sub.fe ( 5)
Since it is desired to maintain the temperature of the chill block well below the solidification temperature, it is preferable that p.sub.b is less than p.sub.fe. To determine the individual power dissipations, it is assumed that due to the double primary layout, the magnetic flux density in either the strip specimen or the conveyor belt is equal in strength, and that the fluxes contained per square centimeter of surface are equal, thereby generating a voltage .xi. around a closed loop of for example 4 centimeters in periphery, l. The power dissipation in this loop is then: EQU p.sub.fe =.xi..sup.2 A/l.tau..sub.fe (t) (6)
where .rho..sub.fe is the volume resistivity which is a function of temperature t and cross-sectional area A of the loop which is the product of strip thickness, t.sub.fe, and the loop transverse dimension. Therefore, the ratio of power dissipation in the strip to that in the conveyor is: EQU p.sub.fe /p.sub.b =t.sub.fe .rho..sub.b (t)/.rho..sub.fe (t)t.sub.b ( 7)
where t.sub.b is the conveyor belt thickness and .rho..sub.b (t) is the corresponding volume resistivity as a function of temperature. In practical applications, t.sub.b is usually larger than t.sub.fe, with 1.27 mm (50 mils) being a minimum level, this reduces to: EQU p.sub.fe /p.sub.b .gtoreq..rho..sub.b (t)/.rho..sub.fe (t) (8)
and the temperature dependence of .rho..sub.fe is less pronounced than that of .rho..sub.b. At a belt speed V.sub.r or 22.8 meters/second, .rho..sub.b may change by 66% over a 50 centimeter length whereas .rho..sub.fe will remain nearly constant at 120 micro-ohm-centimeters in the range of 1200.degree. C. to 1421.degree. C. (the initial solidification temperature). Specifically, the temperature coefficients of a 2.0 mm (80 mils) thick conveyor, if composed of beryllium-copper, is 0.00393/unit/.degree.C. for temperatures above 20.degree. C. For example, at a 77.degree. C. initial conveyor temperature, this conductivity is 3.84.times.10.sup.7 /ohm-meter while at a distance of 50 centimeters along the belt, the copper surface temperature is between 900.degree. and 1100.degree. K., indicating a conductivity of 1.3.times.10.sup.7 /ohm-meter or a reduction to 34% of the initial value.
With the resistivities and thicknesses established, the specific force on each must be considered in terms of either Newtons/square meter of surface/watt dissipated in each material or else the maximum Newtons/square meter of surface for a given temperature rise in the belt. In general, the electromagnetic system will either be primary limited in Joule heating or secondary limited in Joule heating, consequently lengthening the solidification distance. It is unusual for any machine, to be both primary and secondary dissipation limited at the same operating point. In the instance of high frequency excitation, primary slot spacing is very close, even for field speeds of 23 meters/second, which necessitates small electrical conductor wire and relatively poor heat transfer out of the primary member of the electromagnetic pump.